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No key was found for the requested taxon, but it is the only child of Hydrangea. Showing where it is keyed below.

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Keyed in multiple places:

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Key to Hydrangeaceae

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1 Woody vine, climbing by aerial rootlets; petals 7-10; [tribe Hydrangeae]
1 Shrub; petals 4-5 (rarely 10 or many in the cultivars of Deutzia and Philadelphus).
  2 Pubescence of leaves and twigs stellate; stamens 10; [a cultivated exotic, rarely escaped]; [tribe Philadelpheae]
  2 Pubescence of leaves and twigs simple; stamens 8-10 (Hydrangea) or 25-90 (Philadelphus); [natives and exotics].
    3 Leaf blades 10-30 cm long; inflorescences of 25-many flowers; stamens 8-10; [tribe Hydrangeae]
    3 Leaf blades 3-8 cm long; inflorescences of 1-7 flowers; stamens 25-90; [tribe Philadelpheae]

Key J2: woody angiosperms with opposite, simple leaves with toothed margins {add [Abelia] CAPRIFOLIACEAE}

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1 Leaves evergreen.
  2 Plant a shrub, erect, not requiring support.
    3 Leaves with spiny margins
    3 Leaves with crenate or serrate margins.
      4 Leaves slightly to strongly fleshy; inflorescence a head; [maritime situations]
 Iva
      4 Leaves not fleshy; inflorescence either a head or otherwise; [collectively widespread].
        5 Leaves > 8 cm long, typically spotted with yellow, coarsely toothed; fruit a red drupe; [commonly cultivated, rarely seeding down nearby]
        5 Leaves < 8 cm long, not yellow-spotted, serrulate; fruit a capsule or purplish drupe; [plants native or cultivated].
          6 Inflorescence otherwise; [more widespread].
             7 Corolla tubular, campanulate, bilaterally symmetrical; fruit a 1-seeded achene (rarely produced)
             7 Corolla radially symmetrical, with 4-5 distinct petals; fruit a 2-10-seeded capsule or drupe.
               8 Fruit a 4-5-locular capsule, with 2 seeds per locule (though often fewer by abortion)
               8 Fruit a (2-) 3-locular, purplish-black drupe, with (2-) 3 single-seeded stones
  2 Plant a subshrub, creeping shrub, or liana.
                 9 Leaves spinose-serrate; [exotics, rarely naturalized]
                 9 Leaves serrate (not spinose), serrulate, or crenate; [exotics and natives, collectively widespread].
                   10 Leaves slightly to strongly fleshy; inflorescence a head; [maritime situations]
 Iva
                   10 Leaves not fleshy; inflorescence otherwise; [collectively widespread].
                     11 Leaves on vigorous shoots with a few coarse rounded teeth towards the base (most leaves entire)
                     11 Leaves serrulate to serrate, the teeth uniformly around the margin or concentrated towards the tip; fruit dry, either indehiscent and 1-seeded or capsular and with several seeds.
                       12 Flowers 5-merous; petals fused; fruit indehiscent, 1-seeded; [montane, from e. TN, WV, and w. MD northwards in our area]
                       12 Flowers 4- or 5-merous; petals separate; fruit capsular, dehiscent, several-seeded; [collectively widespread in our area]
1 Leaves deciduous.
                          13 Leaves slightly to strongly fleshy; inflorescence a head, subtended by an involucre of phyllaries; [maritime situations]
 Iva
                          13 Leaves not fleshy; inflorescence, flower, and fruit structure various, but not with the combination of features as above (sometimes the flowers in a head subtended by bracts, but then with other features differing, such as stamens 4, or green calyx present, or petals separate, or fruit a schizocarp of mericarps, etc.); [collectively widespread].
                              15 Stems not prickly; foliage smooth to variously hairy, but not scabrous.
                                16 Leaves on vigorous shoots with a few coarse rounded teeth towards the base (most leaves entire), the larger leaves < 3 cm wide; lianas climbing by twining; fruit a fleshy berry; flowers 5-merous, with a fused, tubular corolla
                                16 Leaves serrate, the teeth towards the leaf apex, the larger leaves > 4 cm wide; lianas climbing by adventitious roots; fruit a capsule; flowers 7-10-merous, with separate petals
                            14 Upright shrubs or trees, lacking any adaptations for climbing.
                                  17 Trees; leaves often a mix of alternate and opposite.
                                    18 Leaves harshly scabrous on the upper surface; fruit a multiple of achenes; leaf venation pinnate but irregular
                                    18 Leaves not scabrous; fruit a 2-4-seeded drupe; leaf venation neatly pinnate, the lateral veins nearly straight and parallel to one another
                                  17 Shrubs or trees; leaves strictly opposite (or often a mix of alternate and opposite in RHAMNACEAE).
                                       19 Trees; leaves palmately-veined, with 5 or more veins from the base; [rarely naturalizing]
                                       19 Shrubs; leaves either triple-veined from near the base or pinnate-veined; [collectively widespread and common]
                                         20 Leaves strongly triple-veined from at or near the base of the blade, the 2 lateral veins arching towards the tip and rejoining the midvein or nearly so (becoming diffuse before rejoining); petals 4, white; stamens 15-90
                                         20 Leaves pinnate-veined; petals various, not both 4 and white (except sometimes in Hydrangea); stamens 1-15 (except 15-30 in Exochorda in ROSACEAE).
                                           21 Inflorescence more diffuse, with internal axes and pedicels; flowers not both sympetalous and 4-lobed (except in Forsythia and Buddleja); fruit 1-seeded, 2-4-seeded, or 4-many-seeded.
                                               23 Corolla present; flowers larger, in terminal cymes, corymbs, racemes, panicles, or in axillary cymes or fascicles.
                                                    25 Petals separate; stamens 8-10 (-60) (or 4-6 in RHAMNACEAE and Euonymus in CELASTRACEAE).
                                                      26 Flowers 1 and terminal, or many, in terminal panicles or corymbs; stamens 8-10 (-60); stems brown, tan or gray.
                                                    25 Petals fused, at least basally, and often strongly tubular; stamens 2, 4, or 5.
                                                                                    40 Inflorescence various, but more diffuse, the flowers larger (> 5 mm in diameter, except for some flowers in Hydrangea in HYDRANGEACEAE) and loosely arranged (< 5 per cm of axis).